Phe-Met-Arg-Phe-amide immunoreactivity (FMRF-NH2-IR) is highly concentrated in the dorsal horn of rat spinal cord, and particularly in nerve terminals of lamina I. In order to establish the location of the cell bodies of the lamina I terminals containing FMRF-NH2-IR in sensory ganglia and in spinal roots. FMRF-NH2-IR was found in both tissues, and reverse-phase HPLC analysis revealed that both tissues contain the same molecular forms that are also present in the spinal cord. Lumbo-sacral rhizotomy induced a 50% decrease of FMRF-NH2-IR in the lumbar segment of the spinal cord, suggesting that at least a portion of the FMRF-NH2-IR present in this tissue is of peripheral origin. Transection of the spinal cord at the midthoracic level induced a 20 to 50% decrease of FMRF-NH2-IR in the lumbar segment of the spinal cord, suggesting also that the presence of FMRF-NH2-IR in descending pathways.